The present invention relates to the infrared detection of hydrocarbon gases (which term includes vapours).
The use of non-dispersive infrared spectroscopy to detect gases is well established. It essentially involves transmitting infrared radiation along a path in an area being monitored; the wavelength of the infrared radiation is chosen so that it is absorbed by the gas of interest (hereafter called the xe2x80x9ctarget gasxe2x80x9d) but not substantially absorbed by other gases in the atmosphere of the area being monitored. If monitoring out-of-doors, the wavelength should ideally not be absorbed by liquid water (e.g. in the form of condensation, rain or spray). The intensity of the radiation that has passed along the path in the area being monitored is measured and the attenuation in the intensity of the radiation gives a measure of the amount of the target gas in the monitored area.
However, factors other than absorption by the target gas also attenuate the infrared radiation including obscuration of the detecting beam, atmospheric scattering of the radiation, contamination of optical surfaces, e.g. by dirt or condensation, and ageing of components. The reliability of infrared gas detectors is significantly improved by the use of a reference; such a reference is usually infrared radiation at a different wavelength, which ideally is a wavelength at which the target gas does not exhibit significant absorption. The ratio between the signal obtained at the wavelength where the target gas does absorb (the xe2x80x9csample wavelengthxe2x80x9d) and the signal obtained at the wavelength where the target gas does not significantly absorb (the xe2x80x9creference wavelengthxe2x80x9d) compensates for the attenuation caused by non-target gases since ideally the signal at the reference wavelength and the signal at the sample wavelength will both be affected by such non-target gas attenuation.
A known infrared detector is a so-called xe2x80x9cfixed pointxe2x80x9d detector, which has a very short path length (e.g. up to 10 cm) and so only monitors a relatively small space. It can be used to detect leakages of hydrocarbons from oilrigs, pipelines, storage tanks or refineries. The provision of such detectors in open spaces away from a leakage site may result in the leakage not being detected since prevailing atmospheric conditions (e.g. wind speed, wind direction and temperature) could carry the gas away from the detectors, which would then not register the leakage. It is therefore a difficult task to position such fixed point detectors and usually a compromise is drawn in the location of detectors, based on likely leak sites and typical prevailing weather conditions; also the number of such detectors that can be provided is limited by cost. Generally fixed-point detectors are used for monitoring of specific items of equipment and apparatus that are liable to leak, for example pipeline joints and valves.
Fixed-point detectors are coupled with an alarm that indicates the detection of a target gas in the immediate neighbourhood of the detector. Because such detectors are placed near the source of any leak, any significant leaking target gas will be in relatively high concentration in and around the detector. It is therefore possible to set the alarm such that the amount of the target gas present before the alarm is triggered is relatively large, thereby avoiding the giving of false alarms. The giving of false alarms is a substantial problem since it could result in the shutting down of a facility, for example an oil rig or an oil refinery.
To overcome the above-mentioned shortcomings of fixed-point detectors, longer path-length gas detectors, so called xe2x80x9copen-path optical gas detectorsxe2x80x9d, are used, in which radiation at sample and reference wavelengths is transmitted along an open-path which passes through the atmosphere in the space to be monitored. The length of the path can vary from one to a thousand meters, depending on the application, and so allows a much greater space to be monitored than is the case with fixed-point detectors. When used out-of-doors, the open nature of the optical path means that the beam is exposed to prevailing atmospheric weather conditions, which can seriously affect the operation of the instrument. For example, rain, snow, mist, fog, sea spray, blizzards and sand or dust storms scatter or absorb radiation at the reference and sample wavelengths. The level of absorption and scattering by such weather conditions depends on the size, shape, nature and optical properties of the droplets, drops or particles concerned. Unfortunately such attenuation is not uniform across the infrared spectrum, i.e. the attenuation at the sample wavelength and the attenuation at the reference wavelength are not identical which gives rise to errors in the measurement of the amount of target gas and can, in extreme cases, lead to the failure to trigger an alarm or the triggering of a false alarm. The matter is complicated considerably because different weather conditions exhibit different relative and absolute attenuation at the sample and reference wavelengths. For example, one sort of fog can attenuate the radiation at the reference wavelength more than the radiation at the sample wavelength whereas a different sort of fog will attenuate the radiation at the sample wavelength more than the radiation at the reference wavelength. The variability of atmospheric attenuation for different weather conditions makes it very difficult to compensate for the effects of weather upon this sort of gas detector.
In order to minimise the differential attenuation between the sample wavelength and the reference wavelength, it is preferable that the two wavelengths are as close as possible to each other. However, this is not always possible since there may not be a suitable reference wavelength, i.e. a wavelength at which the target gas is only minimally absorbed, near the sample wavelength. The situation is made even more complex because of the need to avoid cross-sensitivity to other atmospheric gases, the absorption/refraction characteristics of water droplets that may be present in the path of the infrared beam and the band shapes and tolerances of the filters used to restrict the sample and reference wavelengths. Thus there may be several hundred nanometers between the sample and reference wavelengths. This separation can result in significant differences between the absorption characteristics of the sample and reference wavelengths under different weather conditions, as set out above.
Instead of measuring the reference signal at a single wavelength, it has been proposed (see for example GB-1,402,301, GB-1,402,302, U.S. Pat. No. 4,567,366, EP-0,744,615 and GB-2,163,251) to use two reference wavelengths located on either side of a sample wavelength and take, as the reference signal, the average of the signals at the two reference wavelengths. This arrangement requires the measurement of light absorption at two different reference wavelengths, which in turn requires either the use of separate light beams to measure the absorption at each of the two reference wavelengths or the use of a mechanical arrangement to bring two filters into alignment with a single light-sensitive detector. Both solutions will work satisfactorily in a laboratory but not in the field, particularly not in the harsh environments encountered on offshore oil/gas platforms or in the Middle East, the Tropics, the Arctic, etc. The use of two reference light beams (in addition to the sample light beam) requires careful alignment (within micron tolerances) of the detectors and it is difficult enough to align the detectors for the sample beam and a single reference beam, let along aligning an additional detector for a second reference beam. Furthermore, the buffeting of the detectors in the environment of the North Sea, for example, can displace the alignment. In addition, the use of an additional reference beam makes the system expensive. The use of a mechanical arrangement (e.g. a spinning filter wheel) to bring sample and reference filters periodically into alignment with a single light-sensitive detector is also not feasible in the field since vibrations can affect the operation of the mechanical arrangement and such mechanical parts can be unreliable in the harsh environmental conditions encountered.
It is also known to use as the reference a broad range of wavelengths, and if the range includes the sample wavelength, it is possible to make the average reference wavelength equal to the sample wavelength, thereby eliminating the problems discussed above of having the reference and sample wavelengths distant from each other. However, the inclusion in the reference signal of a substantial component made up of the sample signal itself leads to substantially reduced sensitivity.
All the C1-7 alkanes are gaseous or volatile and their escape can give rise to a risk of an explosion and therefore it is necessary to monitor for the presence of any of them. It is not feasible to provide separate systems for detecting each alkane and therefore it is necessary that a single system should be capable of detecting all these alkanes. However, the alkanes all have different spectra and so it is difficult to select a single sample wavelength and a single reference wavelength that can be used for all of the C1-7 alkanes.
A further substantial problem underlying the use of this type of detector is the need to avoid false alarms being given, which can result in the shut down of a complete facility, for example oil pipeline, oil refinery or oil rig. It is obviously desirable to be able to detect the smallest possible concentration of the target gas that could give rise to a hazard. However, this must be set against the need to avoid false alarms and the above-described problem of atmospheric conditions differentially affecting the reference and sample wavelengths.
It is an object of the present invention to provide an infrared detector of the open path type described that is capable of monitoring for all, of the C1-7 alkanes and yet is sufficiently rugged that it can be operated reliably in the field in harsh environments. It is a further object of the present invention to provide an infrared detector that will have an improved accuracy for detecting alkanes in a variety of harsh weather conditions as compared to known open path infrared detectors so that the instances of false alarms are reduced.
According to the present invention there is provided an infrared gas detector for a target gas comprising:
an infrared source capable of transmitting at least one infrared beam comprising radiation in at least one wavelength band at which the target gas is absorbent (sample wavelength band) and in at least one wavelength band at which the target gas is only absorbent to a substantially lesser extent than at the sample wavelength band (reference wavelength band);
a first radiation intensity sensor capable of sensing the intensity of the infrared radiation in the sample wavelength band(s) and a second radiation intensity sensor capable of sensing the intensity of the infrared radiation in the reference wavelength band(s), both of which are spaced apart from the transmitter by a beam path; and
a first filter that is located in front of the first sensor and that only transmits one or more sample wavelength bands and a second filter that is located in front of the second sensor and that only transmits one or more reference wavelength bands, wherein the aggregate number of sample and reference wavelength bands transmitted by the two filters is at least three and wherein the centre of the sample wavelength band (if there is only one sample wavelength band) or the mid point between the sample wavelength bands (if there are multiple sample wavelength bands) is approximately the same as the centre of the reference wavelength band (if there is only one reference wavelength band) or the mid point between reference wavelength bands (if there are multiple reference wavelength bands).
By making the mid-point of the reference wavelenlgth band(s) approximately the same as the mid-point of the sample wavelength band(s), the differential attenuation of radiation at the sample and reference wavelengths caused by atmospheric conditions can be eliminated or substantially reduced. As indicated, the aggregate number of sample and reference wavelength bands transmitted by the two filters can exceed three so long as the mid point between the bands at the sample wavelengths is approximately the same as the mid point of the bands at the reference wavelengths, although exact coincidence of the two mid points is not required.
A single infrared beam emitted by the transmitter can contain the sample and the reference wavelength bands or separate beams may be provided. Indeed, the beam emitted by the transmitter can include a wide spectrum of wavelengths, not only the sample and reference wavelength bands. A filter may be inserted in the infrared beam in order to transmit along the detection path only wavelengths around the wavelengths detected by the detector sensor(s) but such a filter is not necessary. The use of two separate radiation intensity sensors, one for the sample wavelength band(s) and the other for the reference wavelength band(s) means that there are only two sensors in the system that need aligning and there is no need to move the first and second filters.
In order to discriminate between infrared radiation from the transmitter and infrared radiation from other potentially interfering sources of radiation, e.g. sunlight, the radiation from the transmitter is preferably modulated with a distinct characteristic that can be recognised by the receiver; this modulation can be achieved in many ways, including pulse or amplitude modulation of the infrared source""s drive voltage/current, acousto-optic modulation and electro-optic modulation. In the preferred embodiment, the modulation is achieved by pulsing the voltage applied to a flashlamp, e.g. a Xenon arc flashlamp. This pulsing produces short, very high intensity pulses that are easily discriminated from both natural and artificial sources that are likely to be encountered in the intended operating environment.
One highly important aspect of the present invention is the selection of the sample and reference wavelength bands that allow the reliable detection of C1-7 alkanes. The first (sample) filter advantageously transmits a single band having a central wavelength of 2300xc2x15 nm and a full width half maximum (FWHM) of 50xc2x110 nm. The second (reference) filter is preferably a dual bandpass filter having a first band centred around 2215xc2x15 nm with a FWHM of 25xc2x15 nm and a second band having a central wavelength of 2385xc2x15 nm and a FWHM of 25xc2x15 nm. It can be seen that the sample wavelength band lies mid way between the two reference wavelength bands of the dual bandpass reference filter. The selection of these wavelengths is not apparent from the spectra of C1-7 alkanes since some of the alkanes are significantly absorbent at the reference wavelengths and yet, as shown below, we have surprisingly shown that the choice of these wavelengths provides good detection of C1-7 alkanes and avoids giving false alarms even in the presence of rain and fog.